Question: In math class, the girl to boy ratio is $3$ to $6$. If there are $30$ boys, how many girls are there?
A ratio of $3$ girls to $6$ boys means that there is a group of $3$ girls for every group of $6$ boys. If there are $30$ boys, then there are $5$ groups of $6$ boys each. Because we know the ratio is $3:6$ , there must be $5$ groups of $3$ girls each. There is a total of $15$ girls in math class.